In recent years, as machines are increasingly reduced in weight and size, the rolling bearings used for the machines are also reduced in size. Accordingly, there is a tendency that contact pressure caused in the rolling bearing (or between a race member and a rolling element) increases. This results in increased temperature, and hence a lubricant degraded and reduced in viscosity. This in turn provides an insufficient oil film, which may result in the rolling element and the race member (an inner ring and an outer ring) having their respective surfaces in metal-to-metal contact with each other and thus increasingly worn. In particular, under such a condition of lubrication that an oil film parameter Λ represented by the following expression:Λ=h0/(α12+α22)1/2  (1)has a value less than one, wear is a significant issue, wherein h0 represents the oil film in thickness and α12 and α22 represent a raceway surface and the rolling element, respectively, in surface roughness RMS.
When the race member's raceway surface, the rolling element's surface (that contacts the race member) and the like are worn and thus have projections and depressions, the projections and depressions serve as a source of stress concentration or dropping iron powder is bit therein, and the rolling bearing may be reduced in lifetime.
One application in which a rolling bearing is disadvantageously worn, as described above, is for example a self-aligning roller bearing used for a guide roll of equipment operated to continuously cast steel. The guide roll of such equipment receives melted steel's weight and is exposed to heat of high temperature and thus thermally expands, warps, and/or the like. Accordingly, the guide roll is often supported by a rolling bearing (hereinafter referred to as a guide roll bearing) implemented as a self-aligning roller bearing that can tolerate an inner ring's aligning and also has a high ability to withstand axial load. The guide roll bearing is used under a condition with an inner ring rotating at an extremely low rate and a large load exerted, and thus has an oil film hardly formed and is worn at a raceway surface of an outer ring with a fixed load area. In particular, a self-aligning roller bearing has rollers in the form of barrels and it is thus known to cause differential slip, and a wear difference is caused between a simply rolling portion and a sliding portion and an outer ring raceway surface is partially worn forming two ridges. The partial wear results in two projections, at which stress concentration arises, and can be a cause of early exfoliation. Furthermore, melted steel is solidified by coolant water jetted thereto in a large amount, which causes water, vapor and the like to enter the bearing. A guide roll bearing used in such a severe condition of lubrication is required to prevent/minimize wear of its bearing components (such as a race member, a rolling element, and the like).
Furthermore, a steel configuring a bearing component of a bearing used in a high fit to a shaft is implemented by a carburized carburizing steel capable of providing a surface layer with compressive residual stress for the purpose of preventing tensile stress from reducing lifetime and cracking an inner ring. Carburized carburizing steel is excellent in fracture toughness at its core is smaller in hardness than its surficial carburized portion, and is thus more resistive to cracking than uniformly hardened steel (such as JIS SUJ2). Guide roll bearings are also formed universally with carburizing steel to avoid suddenly fracturing and prevent a roll from accordingly dropping off.
Furthermore, a carbonitriding process may be utilized as a means for enhancing wear resistance and increasing a bearing's lifetime while the bearing is provided with lubrication with foreign matters introduced therein. The carbonitriding process also effectively reduces/prevents a matrix's reduction in hardness at high temperature, and is used widely not only for a rolling bearing for equipment operated to continuously cast iron steel but also a rolling bearing for rolling equipment, a rolling bearing for a dryer roll of a paper making machine, and the like. However, these bearings are required to have further longer lifetime and simply performing a carbonitriding process does not ensure sufficient wear resistance.
Including conventional art as described above, two approaches have mainly been proposed to improve a bearing component in wear resistance. A first approach is to improve surface hardness. Surface hardness can be improved for example by the above carbonitriding process. While originally a bearing component of steel is quenched to be increased in hardness and is thus also excellent in wear resistance, a nitriding process can further increase its surface hardness and hence further improve its wear resistance (see Japanese Patent Laying-Open No. 8-311603 (patent literature 1) for example).
A second approach is to increase the amount of precipitates of large hardness. Fine precipitates of large hardness present in a large amount provide increased wear resistance. Production of precipitates of large hardness is significantly affected by an element added in steel. More specifically, adding vanadium (V), aluminum (Al), chromium (Cr), titanium (Ti), molybdenum (Mo) or the like to steel and for example carbonitriding it provides precipitates of large hardness (see Japanese Patent Laying-Open No. 8-49057 (patent literature 2) for example).